Syntheses of many organic pigments include a coupling step in a dilute aqueous medium to produce a slurry of the pigment product, which is typically followed by a step of filtering the slurry in a filter press to concentrate the pigment. The press cake that results is then either dried to provide a dry, particulate pigment or else is “flushed” with an organic medium such as an oil or resin to transfer the pigment particles from the aqueous press cake to the oil or resin phase. The flushing process requires additional time and materials over simply drying the pigment. If the pigment is used in an ink or coating composition, however, it must first be well-dispersed in an appropriate organic medium in order to achieve the desired color development and stability, and thus the flushing process is advantageous because it accomplishes the transfer without intermediate steps of drying the pigment and grinding the pigment in the organic medium to produce the pigment dispersion.
In the past, pigment flushes have usually been prepared by batch processes in which the press cake is kneaded with an organic phase such as an oil or a resin, for example in a sigma blade mixer or dough mixer, to flush the pigment particles from the water phase to the organic medium phase and displace the water as a separate aqueous phase. The displaced water is decanted and the dispersion of the pigment in the varnish can be used as a pigment paste in preparing an ink or paint.
The batch process has many shortcomings. The steps of adding varnish, kneading the dough to displace the water, and pouring off the water must usually be repeated a number of times in order to obtain the maximum yield from the flusher. This is a labor-intensive process that requires careful monitoring. Further, in order to remove the residual water, the batch must be heated and stripped under vacuum. For many pigments, the heat history from processing to remove the residual water results in a color shift. Because the precise heat history may vary from batch to batch, it is difficult to reliably and accurately reproduce the same color from batch to batch. One of the most significant shortcomings of the batch process is the lot to lot fluctuations of the pigment and the solids content of the presscake. Further, the process is time-consuming and inefficient. It is difficult to reduce the water content below about 3% by weight, even with the vacuum stripping. Finally, the pigment content of presscake, and therefore of the flush can vary.
Continuous processes for preparing pigment flushes using presscake have been proposed. In particular, the processes described in U.S. Pat. Nos. 6,273,599, 6,305,838, and 6,348,091, each of which is incorporated herein by reference, have been highly successful in resolving some of the shortcomings associated with batch flushing processes. U.S. Pat. No. 5,262,268 describes using presscake to produce a toner in a twin-screw extruder.